1. Field of the Invention
The present invention relates to a motor driving technique, and more particularly to a driving apparatus and a method that drive a motor in a position sensor-less system to detect a rotor position based on a zero-cross point of a back electromotive force generated in a non-conduction phase of windings.
2. Related Art
In recent years, a brush-less motor has been generally used as a spindle motor for a hard disk, an optical disk and the like, or as a fan motor and a compressor-driving motor for an air conditioner. In general, the brush-less motor is driven in PWM control by an inverter so as to provide a variable speed controlling operation in a wide range and also to achieve low power consumption.
Inside the brush-less motor having three-phase windings, in general, position sensors such as hole elements are placed at every electrical angle of 120 degrees so as to detect the position of a magnetic pole of a rotor. Here, in order to reduce costs and achieve a small-size device, various sensor-less techniques have been developed. With respect to a means for achieving a sensor-less driving operation, there is a method including the steps of carrying out 120 degree conduction, and detecting the rotor position by detecting a zero-cross point of a back electromotive force generated during a non-conduction period. In this method, however, since no back electromotive force is generated unless the motor is rotating, the back electromotive force is not detected at the time of starting the motor, with the result that a starting failure, such as oscillation, loss of synchronism and reverse rotation, tends to occur depending on the initial position of the rotor.
For this reason, a means for determining a relative position of the rotor to the stator in a motor stop state has been proposed.
For example, Japanese Patent No. 2547778 has proposed a controlling system in which a voltage is applied across each terminal of stator windings for such a short period of time as not to allow the rotor to react to move so that the winding that generates an electric current having the highest amplitude value is determined as a winding to be used for starting a current supply.
Moreover, for example, JP2001-275387A has proposed a controlling system in which a current pulse, which is such a short pulse as not to allow the rotor to react to move, is allowed to successively flow each terminal with its polarity being changed, so that an induced voltage generated in a non-conduction phase at this time, is detected and added to determine the initial position of the rotor. FIG. 19 is a drawing that shows a structure used for achieving the controlling system indicated by JP2001-275387A. FIG. 20A is a drawing that shows the results of measurements of the induced voltage generated by the current pulse generated in the non-conduction phase.
In FIG. 19, a motor 10 includes a rotor (not shown) having a magnetic-field section given by a permanent magnet and a stator in which a U-phase winding 11, a V-phase winding 12 and a W-phase winding 13 are Y-connected. A current output unit 20 is composed of, for example, 3-phase bridge circuits and is placed between a power supply and a GND so that a voltage is applied to the terminal of each winding to flow a current. A phase-switching controller 50 determines a winding through which a current pulse flows, and outputs a signal for providing a current pulse flow to the current output unit 20 and a signal for indicating a selected winding to an induced voltage detection unit 110, respectively.
In response to the signal of the phase-switching controller 50, the induced voltage detection unit 110 detects induced voltages of respective phases from three-phase terminal voltages Vu, Vv and Vw as well as a neutral-point voltage Vc of the motor 10, and outputs values of the induced voltages to an adder 120. The adder 120 adds an induced voltage of a non-conduction phase obtained when current pulses forwardly flows through a phase winding to an induced voltage thereof obtained when current pulses reversely flows through the phase winding.
For example, in FIG. 20A, a solid line 111 indicates an induced voltage generated in the W-phase winding 13 upon supplying current pulses from the U-phase winding 11 to the V-phase winding 12, and a broken line 112 indicates an induced voltage generated in the W-phase winding upon supplying current pulses from the V-phase winding 12 to the U-phase winding 11 in a reversed manner. The ordinate axis represents a detection voltage (mV), and the abscissa axis indicates a relative position of the rotor to the stator with an electrical angle (degrees). The adder 120 adds the induced voltage 111 and the induced voltage 112 detected by the induced voltage detection unit 110 and shown in FIG. 20A to obtain a sum of induced voltages 113 shown in FIG. 20(b).
A polarity determining unit 130 determines the polarity of the sum 113 of induced voltages obtained in the adder 120. When it is determined to be the sum of induced voltages of U-phase, the unit 130 outputs the resulting signal to a UDATA storage device 141. When it is determined to be the sum of induced voltages of V-phase, the unit 130 outputs the resulting signal to a VDATA storage device 142. When it is determined to be the sum of induced voltages of W-phase, the unit 130 outputs the resulting signal to a WDATA storage device 143. In FIG. 20B, since the sum of induced voltages of W-phase is shown, the determined polarity is output to the WDATA storage device 143. Moreover, with respect to the U-phase and V-phase, the sum of induced voltages is obtained in the same manner, and the determined polarity is output to the UDATA storage device 141 and the VDATA storage device 142, respectively. Each of the UDATA storage device 141, the VDATA storage device 142 and the WDATA storage device 143 stores the polarity of the sum of induced voltages output from the polarity determining unit 130.
A judgment unit 150 judges the initial position of the rotor based on combinations of polarities respectively stored in the UDATA storage device 141, the VDATA storage device 142 and the WDATA storage device 143.
A timing generator 160 outputs timing signals to the phase-switching controller 50, the induced voltage detection unit 110, the adder 120, the UDATA storage device 141, the VDATA storage device 142, the WDATA storage device 143 and the judgment unit 150 so as to control timings of the respective processes.
In the control system proposed in Japanese Patent No. 2547778, since the highest amplitude value is varied dependent on production deviations among the windings of the stator, a detection error tends to occur due to slight deviations in the windings. Consequently, a starting failure, such as oscillation, loss of synchronism and reverse rotation, tends to occur.
Moreover, in the control system proposed in JP2001-275387A, an adder 120, which holds induced voltages and adds these, is installed, and an externally added capacity and circuits for charging and discharging the capacity are required as constituent components. Moreover, since the rotor initial position is judged based on combinations of polarities of the sum of induced voltages of the respective phase windings, it is necessary to refer to a table or the like relating to the combinations, which makes the structure of the device more complicated. Moreover, since an induced voltage is detected by using a pair of current pulses in a forward direction and a reverse direction, and since the induced voltage is detected for each of the phase windings, current pulses of six patterns in total need to be always supplied. For this reason, upon judging the initial position of the rotor, a period of time (one cycle) required for supplying the current pulses of six patterns is inevitably required.